User IDs in the URL are better in my opinion, since they are limitless (because names have limits, and numbers don't), but it also makes a hard time for people to type out a certain user's webpage URL.
In the meanwhile, Usernames in the URL aren't good for a site that receives a lot of incoming users. It might give people an easy time typing an user's URL, but there's a certain limit to the number of users that this system can hold. (Note: Limit depends on the maximum number of characters allowed per username.)
It's up to you, Tom. You could make a poll or something like that, or sort out the problem yourselves. I hope you could sail through this dilemma safely.